|YOU'VE FORGOTTEN YOUR PURPOSE!
Aku City was a city that was the main location in the future in several episodes.
Aku at some point has established this city some time after he conquered much of the world. Because of mountains of tributes that flowed in from around the world in name of Aku, the city has became the wealthiest settlement on Earth. Aku centralized the bulk of his military industrial complex within the city, also making it a technological and commercial hub. People from all over the world flocked to the city in search of employment and residency, having to completely submit to Aku's rule. Because of this, the city was not only the world's political capital akin to the United Nations, but also its largest population center. However, it quickly became a hotbed for a resistance movement against Aku. Aku would rectify this issue by mobilizing his scientific advisors to develop automated drones to hunt down and eliminate agitators. It took time, but eventually the scientists produced the X-Series which were incredibly effective at purging subversive cells, causing severe collateral damage to the city. Eventually, all but one of the X-Series were destroyed and the resistance was either broken or driven out of the city. Absent of oppositions and impatient with the extent of his conquests, Aku commissioned his city as a spaceport, sending his forces out into interstellar expeditions to raid foreign planets of their resources and people. Over the next millennia, the Aku City was the predominant entry point on Earth for the masses of natural commerce and migration as well as forceful tribute and enslavement. Humans quickly found themselves as a minority among many species of aliens arriving on Earth and robots being built. In this time the Aku City rapidly expanded into a super metropolis.
When Jack was first sent into the future, he arrived in the Aku City. Jack would return to the city several times to upset structures of Aku's regime, resupply for his travels, expand his contacts and seek time portals. The city was presumably erased from existence or altered after the destruction of the past Aku.
Aku City is for the lack of a better word humongous. The urban sprawl likely covers an entire continent and the skyscrapers reach miles into the sky. Millions, perhaps billions of humans, aliens and robots call the city home. The city contains thousands of factories, stores, office buildings, apartments and other facilities. The city has indisputably the most advanced level of technology on Earth with flying cars, spacecraft, automated systems, consumer electronics and an unyielding amount of electricity. All businesses and workers within the city are required to pay tribute to Aku, as well as include his name in their branding. This has effectively placed Aku in charge of the city's communications and media of all types. Dominating the city's skyline is the spaceport where interstellar trade and tribute pour in through, as well as intergalactic criminals seeking refuge on Earth. While some in the city are incredibly rich because of its center of commerce, Aku's endless greed has left many in the city impoverished. The lower levels of the city are largely neglected as Aku and his administrators simply do not care about its foundation. These areas are crime ridden, trash laden and increasingly abandoned. The citizens of Aku City generally speak in slang as a confluence of various languages spoken in its maze.
It is unclear where this city was located. A possible location is somewhere in the Japanese archipelago, since Aku originated there. Aku has enslaved the Japanese to mine the resources of their land so that he may spread his evil across the world, not to mention constructing numerous totems in his likeness as well. His lair that housed his seat of power was also situated somewhere in Japan since Jack travelled there on horseback. The city name contained Aku's name which is Japanese for evil, so its name mean Evil City, which was apt given the city's corrupted nature.
In Episode XCVI, after showing Ashi the forest that was once destroyed by Aku, they travelled to the city that was presumably a few distance away. Since the forest is the same one in Episode CI, so the city is indeed in Japan.